One document matched: draft-ietf-dane-srv-01.xml
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<rfc ipr="trust200902"
category="std"
docName="draft-ietf-dane-srv-01">
<!-- === -->
<front>
<title abbrev="TLSA and SRV">
Using DNS-Based Authentication of Named Entities (DANE)
TLSA records with SRV and MX records.
</title>
<author initials="T." surname="Finch" fullname="Tony Finch">
<organization abbrev="University of Cambridge">
University of Cambridge Computing Service
</organization>
<address>
<postal>
<street>New Museums Site</street>
<street>Pembroke Street</street>
<city>Cambridge</city>
<code>CB2 3QH</code>
<country>ENGLAND</country>
</postal>
<phone>+44 797 040 1426</phone>
<email>dot@dotat.at</email>
<uri>http://dotat.at/</uri>
</address>
</author>
<date month="February" year="2013"/>
<area>Security</area>
<workgroup>DNS-Based Authentication of Named Entities (DANE)</workgroup>
<abstract>
<t>The DANE specification [RFC6698] describes how to use TLSA
resource records in the DNS to associate a server's host name with
its TLS certificate. The association is secured with DNSSEC. Some
application protocols can use SRV records [RFC2782] to indirectly
name the server hosts for a service domain. (SMTP uses MX records
for the same purpose.) This specification gives generic
instructions for how these application protocols locate and use
TLSA records. Separate documents give the details that are
specific to particular application protocols.</t>
</abstract>
</front>
<middle>
<!-- === -->
<section anchor="intro" title="Introduction">
<t>The base DANE specification <xref target="RFC6698"/> describes
how to use TLSA resource records in the DNS to associate a
server's host name with its TLS certificate. The association is
secured using DNSSEC. That document "only relates to securely
associating certificates for TLS and DTLS with host names" (see
the last paragraph of section 1.2 of
<xref target="RFC6698"/>).</t>
<t>Some application protocols do not use host names directly, but
instead use a service domain. The domain's servers are located
indirectly via SRV records <xref target="RFC2782"/> (or MX records
in the case of SMTP <xref target="RFC5321"/>). When they do not
use host names <xref target="RFC6698"/> does not direcly apply to
these protocols.</t>
<t>This document describes how to use DANE TLSA records with SRV
and MX records. To summarize:
<list style="symbols">
<t>We rely on DNSSEC to secure the association between the
service domain and the target server host names, i.e. the
result of the SRV or MX query.</t>
<t>The TLSA records are located using the SRV port, protocol, and
target host name fields.</t>
<t>Clients always use TLS when connecting to servers with TLSA
records.</t>
<t>The server's certificate is expected to authenticate the
server host name, rather than the service domain.</t>
</list></t>
<t>Separate documents give the details that are specific to
particular application protocols. For examples,
see <xref target="I-D.ietf-dane-smtp"/>
and <xref target="I-D.ietf-dane-mua"/>.</t>
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
memo are to be interpreted as described in <xref target="RFC2119"/>.</t>
</section>
<!-- === -->
<section anchor="dns" title="DNS checks for TLSA and SRV / MX records">
<section title="MX records">
<t>For the purpose of this specification (to avoid cluttering the
description with special cases) each MX record
(<xref target="RFC5321"/> section 5) is treated as being
equivalent to a SRV record <xref target="RFC2782"/> with
corresponding fields copied from the MX record and the remaining
fields having fixed values as follows:
<list style='hanging'>
<t hangText="Service -">smtp</t>
<t hangText="Proto -">tcp</t>
<t hangText="Name -">MX owner name (mail domain)</t>
<t hangText="TTL -">MX TTL</t>
<t hangText="Class -">MX Class</t>
<t hangText="Priority -">MX Priority</t>
<t hangText="Weight -">0</t>
<t hangText="Port -">25</t>
<t hangText="Target -">MX Target</t>
</list></t>
<t>For example this MX record is treated as if it were the
following SRV record:
<figure><artwork><![CDATA[
example.com. 86400 IN MX 10 mx.example.net.
_smtp._tcp.example.com. 86400 IN SRV 10 0 25 mx.example.net.
]]></artwork></figure></t>
<t>Other details that are specific to SMTP are described in
<xref target="I-D.ietf-dane-smtp"/>.</t>
</section>
<section title="SRV query">
<t>When the client makes a SRV query, a successful result will be
(a possible chain of CNAME / DNAME aliases referring to) a list of
one or more SRV records.</t>
<t>For this specification to take effect, all of these DNS RRsets
MUST be "secure" according to DNSSSEC validation
(<xref target="RFC4033"/> section 5). In the case of aliases, the
whole chain MUST be secure as well as the ultimate target. (This
corresponds to the AD bit being set in the response(s) - see
<xref target="RFC4035"/> section 3.2.3.)</t>
<t>If they are not all secure, this protocol has not been fully
deployed. The client SHOULD fall back to its non-DNSSEC non-DANE
behaviour. (This corresponds to the AD bit being unset.)</t>
<t>If any of the responses is "bogus" according to DNSSEC
validation the client MUST abort. (This usually corresponds to a
"server failure" response.)</t>
<t>In the successful case, the client now has an authentic list of
server host names with weight and priority values. It performs
server ordering and selection using the weight and priority
values without regard to the presence or absence of DNSSEC or
TLSA records. It takes note of the DNSSEC validation status of
the SRV response for use when checking certificate names (see
section <xref target="tls"/>).</t>
</section>
<section title="TLSA queries">
<t>This sub-section applies to each server host name individually,
provided the SRV response was secure according to DNSSEC
validation.</t>
<t>The client SHALL construct the TLSA query name as described in
<xref target="RFC6698"/> section 3, based on fields from the SRV
record: port from the SRV RDATA, protocol from the SRV query
name, and the TLSA base domain is the SRV target host name.</t>
<t>For example this SRV record leads to the following TLSA query:
<figure><artwork><![CDATA[
_imap._tcp.example.com. 86400 IN SRV 10 0 143 imap.example.net.
_143._tcp.imap.example.net. IN TLSA ?
]]></artwork></figure></t>
<t>The client SHALL determine if the TLSA record(s) are usable
according to section 4.1 of <xref target="RFC6698"/>. This
affects SRV handling as follows:</t>
<t>If the TLSA response is "secure" the client MUST use TLS when
connecting to the server. If the client receives zero usable
certificate associations, it processes TLS in the normal fashion
without any input from the TLSA records. If the client receives
one or more usable certificate associations, it processes them as
described in <xref target="RFC6698"/>.</t>
<t>If the TLSA response is "insecure" or "indeterminate" the
client SHALL proceed as if this server has no TLSA records. It
MAY connect to the server with or without TLS.</t>
<t>If the TLSA response is "bogus" then the client MUST NOT
connect to the corresponding server. (The client can still use
other SRV targets.)</t>
</section>
</section>
<!-- === -->
<section anchor="tls" title="TLS checks for TLSA and SRV / MX records">
<t>When connecting to a server, the client MUST use TLS if the
responses to the SRV and TLSA queries were "secure" as described
above. The client SHALL ensure the server's certificate passes the
<xref target="RFC6698"/> checks if there are usable TLSA
records.</t>
<t>The client uses the DNSSEC validation status of the SRV query in
its server certificate identity checks. (The TLSA validation
status does not affect the server certificate identity checks.) It
MUST use the Server Name Indication extension (TLS SNI)
<xref target="RFC6066"/> with the preferred name chosen as
follows. It SHALL verify the identity asserted by the server's
certificate according to <xref target="RFC6125"/> section 6, using
a list of reference identifiers constructed as follows.
<list style="hanging">
<t hangText="SRV is insecure or indeterminate:">
The reference identifiers SHALL include the service domain and
MUST NOT include the SRV target host name. The service domain is
the preferred name for TLS SNI.</t>
<t hangText="SRV is secure:">
The reference identifiers SHALL include both the service domain
and the SRV target host name. The target host name is the
preferred name for TLS SNI.</t>
</list></t>
<t>(In the latter case, the client will accept either identity so
it is compatible with servers that do and do not support this
specification.)</t>
</section>
<!-- === -->
<section title="Guidance for application protocols">
<t>Separate documents describe how to apply this specification to
particular application protocols. If you are writing such as
document the following points ought to be covered: (This section
is currently sketchy.)
<list style="symbols">
<t>SRV fallback logic? In the event of bogus replies etc.</t>
<t>Compatibility with non-SRV clients.</t>
</list>
</t>
</section>
<!-- === -->
<section title="Guidance for server operators">
<t>In order to support this specification, server software MUST
implement the TLS Server Name Indication extension (TLS SNI)
<xref target="RFC6066"/> for selecting the appropriate
certificate.</t>
<t>A server that supports TLS and is the target of a SRV record
MUST have a TLS certificate that authenticates the SRV query
domain (i.e. the service domain, or "source domain" in
<xref target="RFC6125"/> terms). This is necessary for clients
that cannot perform DNSSEC validation. This certificate MUST be
the default that is presented if the client does not use TLS
SNI.</t>
<t>In order to support this specification, the server SHOULD also
have a certificate that authenticates the SRV target domain (the
mail server hostname). This can be done using a multi-name
certificate or by using the client's TLS SNI to select the
appropriate certificate. The server's TLSA record SHOULD
correspond to this certificate.</t>
<t>Note: In some application protocols, there are old non-SRV
clients that expect a server's TLS certificate to authenticate its
host name; they are also unlikely to support SNI. This means that
servers for old clients need a different default certificate from
servers that are the targets of SRV records. If the server does
not have a certificate that authenticates all relevant names, it
is necessary to segregate old and new clients. This can be done by
using different target hosts or non-standard ports in the SRV
targets. (The latter avoids the need for additional
certificates.)</t>
</section>
<!-- === -->
<section anchor="security" title="Security considerations">
<section title="Mixed security status">
<t>We do not specify that clients check that all of a service
domain's server host names are consistent in whether they have or
do not have TLSA records. This is so that partial or incremental
deployment does not break the service. Different levels of
deployment are likely if a service domain has a third-party
fall-back server, for example.</t>
<t>The SRV and MX sorting rules are unchanged; in particular they
have not been altered in order to prioritize secure servers over
insecure servers. If a site wants to be secure it needs to deploy
this protocol completely; a partial deployment is not secure and
we make no special effort to support it.</t>
</section>
<section title="A service domain trusts its servers">
<t>By signing their zone with DNSSEC, service domain operators
implicitly instruct their clients to check their server TLSA
records. This implies another point in the trust relationship
between service domain holders and their server operators. Most
of the setup requirements for this protocol fall on the server
operator: installing a TLS certificate with the correct name, and
publishing a TLSA record under that name. If these are not
correct then connections from TLSA-aware clients might fail.</t>
</section>
<section title="Certificate subject name matching">
<t>Section 4 of the TLSA specification <xref target="RFC6698"/>
leaves the details of checking names in certificates to higher
level application protocols, though it suggests the use of
<xref target="RFC6125"/>.</t>
<t>Name checking might appear to be unnecessary, since DNSSEC
provides a secure binding between the server name and the TLSA
record, which in turn authenticates the certificate. However this
latter step can be indirect, via a chain of certificates. A
usage=0 TLSA record only authenticates the CA that issued the
certificate, and third parties can obtain certificates from the
same CA.</t>
<t>So this specification says that clients check that the server's
certificate matches the server host name, to ensure that the
certificate was issued by the CA to the server that the client is
connecting to. The client always performs this check regardless
of the TLSA usage, to simplify implementation and so that this
specification is less likely to need updating when new TLSA
usages are added.</t>
</section>
<section title="Deliberate omissions">
<t>We do not specify that clients check the DNSSEC state of the
server address records. This is not necessary since the
certificate checks ensure that the client has connected to the
correct server. (The address records will normally have the same
security state as the TLSA records, but they can differ if there
are CNAME or DNAME indirections.)</t>
</section>
</section>
<!-- === -->
<section title="Internationalization Considerations">
<t>If any of the DNS queries are for an internationalized domain
name, then they need to use the A-label form
<xref target="RFC5890"/>.</t>
</section>
<!-- === -->
<section title="IANA Considerations">
<t>No IANA action is required.</t>
</section>
<!-- === -->
<section title="Acknowledgements">
<t>Thanks to Mark Andrews for arguing that authenticating the
server host name is the right thing, and that we ought to rely on
DNSSEC to secure the SRV / MX lookup. Thanks to James Cloos, Ned
Freed, Olafur Gudmundsson, Paul Hoffman, Phil Pennock, Hector
Santos, Jonas Schneider, and Alessandro Vesely for helpful
suggestions.</t>
</section>
<!-- === -->
</middle>
<back>
<!-- === -->
<references title="Normative References">
&rfc2119; <!-- MUSTard MAYonnaise -->
&rfc2782; <!-- DNS SRV -->
&rfc4033; <!-- DNSSEC overview -->
&rfc4035; <!-- DNSSEC protocol -->
&rfc5321; <!-- ESMTP -->
&rfc5890; <!-- IDNA -->
&rfc6066; <!-- TLS SNI -->
&rfc6125; <!-- certificate verification -->
&rfc6698; <!-- DANE TLSA -->
</references>
<references title="Informative References">
<reference anchor="I-D.ietf-dane-smtp">
<front>
<title>DNS-Based Authentication of Named Entities (DANE)
for secure SMTP</title>
<author initials="T" surname="Finch" fullname="Tony Finch" />
<date month="March" year="2013" />
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-dane-smtp" />
</reference>
<reference anchor="I-D.ietf-dane-mua">
<front>
<title>DNS-Based Authentication of Named Entities (DANE)
for POP, IMAP, and message submission</title>
<author initials="T" surname="Finch" fullname="Tony Finch" />
<date month="March" year="2013" />
</front>
<seriesInfo name="Internet-Draft" value="draft-ietf-dane-mua" />
</reference>
</references>
<!-- === -->
<section title="Example">
<t>In the following, most of the DNS resource data is elided for
simplicity.</t>
<t><figure>
<artwork><![CDATA[
; mail domain
example.com. MX 1 mx.example.net.
example.com. RRSIG MX ...
; SMTP server host name
mx.example.net. A 192.0.2.1
mx.example.net. AAAA 2001:db8:212:8::e:1
; TLSA resource record
_25._tcp.mx.example.net. TLSA ...
_25._tcp.mx.example.net. RRSIG TLSA ...
]]></artwork>
</figure></t>
<t>Mail for addresses at example.com is delivered by SMTP to
mx.example.net. Connections to mx.example.net port 25 that use
STARTTLS will get a server certificate that authenticates the name
mx.example.net.</t>
</section>
<!-- === -->
<section title="Rationale">
<t>The long-term goal of this specification is to settle on TLS
certificates that verify the server host name rather than the
service domain, since this is more convenient for servers hosting
multiple domains and scales up more easily to larger numbers of
service domains.</t>
<t>There are a number of other reasons for doing it this way:
<list style="symbols">
<t>The certificate is part of the server configuration, so it
makes sense to associate it with the server name rather than the
service domain.</t>
<t>In the absence of TLS SNI, if the certificate identifies the
host name then it does not need to list all the possible service
domains.</t>
<t>When the server certificate is replaced it is much easier if
there is one part of the DNS that needs updating to match,
instead of an unbounded number of hosted service domains.</t>
<t>The same TLSA records work with this specification, and with
direct connections to the host name in the style of
<xref target="RFC6698"/>.</t>
<t>Some application protocols, such as SMTP, allow a client to
perform transactions with multiple service domains in the same
connection. It is not in general feasible for the client to
specify the service domain using TLS SNI when the connection is
established, and the server might not be able to present a
certificate that authenticates all possible service domains.</t>
<t>It is common for SMTP servers to act in multiple roles, as
outgoing relays or as incoming MX servers, depending on the
client identity. It is simpler if the server can present the
same certificate regardless of the role in which it is to act.
Sometimes the server does not know its role until the client has
authenticated, which usually occurs after TLS has been
established.</t>
</list></t>
<t>This specification does not provide an option to put TLSA
records under the service domain because that would add complexity
without providing any benefit, and security protocols are best
kept simple. As described above, there are real-world cases where
authenticating the service domain cannot be made to work, so there
would be complicated criteria for when service domain TLSA records
might be used and when they cannot. This is all avoided by
putting the TLSA records under the server host name.</t>
<t>The disadvantage is that clients which do not do DNSSEC
validation must, according to <xref target="RFC6125"/> rules,
check the server certificate against the service domain, since
they have no other way to authenticate the server. This means that
Server Name Indication support is necessary for backwards
compatibility.</t>
</section>
<!-- === -->
</back>
</rfc>
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